Thank you for using our product and for your courage tho watch the video. I’ve just watched it myself to understand your questions.

1) If using a 2D face as filter, it needs to be large enough to cover the bright light - which may shine through a window. Place it near to your scene initially to see the effect. Then move it away towards the light source to be filtered out. For each HDRI included on the DVD, there is a QTVR movie that shows you the full scene. This helps you identifying where the bright lights are and gives you hints where the 2D face should be placed to. The farther away the filter from the camera, the larger is has to be. And you want to move it quite a bit away to have it and its shadows out of the camera view.

2) There is a small text file for each HDRI which tells you at what altitude (elevation) the sun is in an outdoor HDRI. The azimuth is always at 0°. If you use the free camera and set all rotation angles to 0°, you will see that it looks straight to the upper edge of the display when looking from above. This is also 0° azimuth. If the HDRI is rotated, you know from the yaw angle where to look for the sun.
It may be simpler to save the camera position, then move the camera around until you see the sun in the HDRI. Then you can create the sphere and move it over the sun.
Since you know from the text file where the sun in the HDRI is, you can also hold down the ctrl+alt key (or alt gr) and double click on the sun roller ball. Then, holding down the same keys, click once in the wireframe. You will see a yellow asterisk, which is the sun. Then, in the sky lab, set the sun azimuth to the same position as the HDRI yaw and sun altitude to the value given for the elevation of the sun in the HDRI. Then move the camera around until you find the yellow star, which should now be over the HDRI sun. This star in the wireframe helps you position the eclipsing sphere.

3) For the sun eclipse, I do it like nature - I use a planet or moon: a sphere. The size depends on how far away from the scene you set the sphere. Keep it default gray for positioning, which helps you in setting the size. Check how the light is on the scene, then move up transparency until you got what you need. If the sun is invisible in your render, you’re done. Otherwise, set ambient colour to white and ambiance up to mask the eclipse. Note that the Ambient colour control in the sky lab is a global control. If that one is black, no matter how you set it in the materials lab, it stays black.
You can indeed use a 2D Disk as well, but it is more difficult to place because you have to take the angle of view of the camera in account so that the camera doesn’t look sideways on the 2D disk. You don’t have to bother about that if you use a sphere.